Re: [PATCH bpf-next 2/6] bpf: add BPF_MAP_DUMP command to dump more than one entry per call

[Yonghong Song <yhs@fb.com>]

On 7/25/19 6:47 PM, Alexei Starovoitov wrote:
> On Thu, Jul 25, 2019 at 6:24 PM Brian Vazquez <brianvv.kernel@gmail.com> wrote:
>>
>> On Thu, Jul 25, 2019 at 4:54 PM Alexei Starovoitov
>> <alexei.starovoitov@gmail.com> wrote:
>>>
>>> On Thu, Jul 25, 2019 at 04:25:53PM -0700, Brian Vazquez wrote:
>>>>>>> If prev_key is deleted before map_get_next_key(), we get the first key
>>>>>>> again. This is pretty weird.
>>>>>>
>>>>>> Yes, I know. But note that the current scenario happens even for the
>>>>>> old interface (imagine you are walking a map from userspace and you
>>>>>> tried get_next_key the prev_key was removed, you will start again from
>>>>>> the beginning without noticing it).
>>>>>> I tried to sent a patch in the past but I was missing some context:
>>>>>> before NULL was used to get the very first_key the interface relied in
>>>>>> a random (non existent) key to retrieve the first_key in the map, and
>>>>>> I was told what we still have to support that scenario.
>>>>>
>>>>> BPF_MAP_DUMP is slightly different, as you may return the first key
>>>>> multiple times in the same call. Also, BPF_MAP_DUMP is new, so we
>>>>> don't have to support legacy scenarios.
>>>>>
>>>>> Since BPF_MAP_DUMP keeps a list of elements. It is possible to try
>>>>> to look up previous keys. Would something down this direction work?
>>>>
>>>> I've been thinking about it and I think first we need a way to detect
>>>> that since key was not present we got the first_key instead:
>>>>
>>>> - One solution I had in mind was to explicitly asked for the first key
>>>> with map_get_next_key(map, NULL, first_key) and while walking the map
>>>> check that map_get_next_key(map, prev_key, key) doesn't return the
>>>> same key. This could be done using memcmp.
>>>> - Discussing with Stan, he mentioned that another option is to support
>>>> a flag in map_get_next_key to let it know that we want an error
>>>> instead of the first_key.
>>>>
>>>> After detecting the problem we also need to define what we want to do,
>>>> here some options:
>>>>
>>>> a) Return the error to the caller
>>>> b) Try with previous keys if any (which be limited to the keys that we
>>>> have traversed so far in this dump call)
>>>> c) continue with next entries in the map. array is easy just get the
>>>> next valid key (starting on i+1), but hmap might be difficult since
>>>> starting on the next bucket could potentially skip some keys that were
>>>> concurrently added to the same bucket where key used to be, and
>>>> starting on the same bucket could lead us to return repeated elements.
>>>>
>>>> Or maybe we could support those 3 cases via flags and let the caller
>>>> decide which one to use?
>>>
>>> this type of indecision is the reason why I wasn't excited about
>>> batch dumping in the first place and gave 'soft yes' when Stan
>>> mentioned it during lsf/mm/bpf uconf.
>>> We probably shouldn't do it.
>>> It feels this map_dump makes api more complex and doesn't really
>>> give much benefit to the user other than large map dump becomes faster.
>>> I think we gotta solve this problem differently.
>>
>> Some users are working around the dumping problems with the existing
>> api by creating a bpf_map_get_next_key_and_delete userspace function
>> (see https://urldefense.proofpoint.com/v2/url?u=https-3A__www.bouncybouncy.net_blog_bpf-5Fmap-5Fget-5Fnext-5Fkey-2Dpitfalls_&d=DwIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=XvNxqsDhRi62gzZ04HbLRTOFJX8X6mTuK7PZGn80akY&s=7q7beZxOJJ3Q0el8L0r-xDctedSpnEejJ6PVX1XYotQ&e= )
>> which in my opinion is actually a good idea. The only problem with
>> that is that calling bpf_map_get_next_key(fd, key, next_key) and then
>> bpf_map_delete_elem(fd, key) from userspace is racing with kernel code
>> and it might lose some information when deleting.
>> We could then do map_dump_and_delete using that idea but in the kernel
>> where we could better handle the racing condition. In that scenario
>> even if we retrieve the same key it will contain different info ( the
>> delta between old and new value). Would that work?
> 
> you mean get_next+lookup+delete at once?
> Sounds useful.
> Yonghong has been thinking about batching api as well.

In bcc, we have many instances like this:
    getting all (key value) pairs, do some analysis and output,
    delete all keys

The implementation typically like
    /* to get all (key, value) pairs */
    while(bpf_get_next_key() == 0)
      bpf_map_lookup()
    /* do analysis and output */
    for (all keys)
      bpf_map_delete()

get_next+lookup+delete will be definitely useful.
batching will be even better to save the number of syscalls.

An alternative is to do batch get_next+lookup and batch delete
to achieve similar goal as the above code.

There is a minor difference between this approach
and the above get_next+lookup+delete.
During scanning the hash map, get_next+lookup may get less number
of elements compared to get_next+lookup+delete as the latter
may have more later-inserted hash elements after the operation
start. But both are inaccurate, so probably the difference
is minor.

> 
> I think if we cannot figure out how to make a batch of two commands
> get_next + lookup to work correctly then we need to identify/invent one
> command and make batching more generic.

not 100% sure. It will be hard to define what is "correctly".
For not changing map, looping of (get_next, lookup) and batch
get_next+lookup should have the same results.
For constant changing loops, not sure how to define which one
is correct. If users have concerns, they may need to just pick one
which gives them more comfort.

> Like make one jumbo/compound/atomic command to be get_next+lookup+delete.
> Define the semantics of this single compound command.
> And then let batching to be a multiplier of such command.
> In a sense that multiplier 1 or N should be have the same way.
> No extra flags to alter the batching.
> The high level description of the batch would be:
> pls execute get_next,lookup,delete and repeat it N times.
> or
> pls execute get_next,lookup and repeat N times.
> where each command action is defined to be composable.
> 
> Just a rough idea.
>